The present invention relates to an apparatus for controlling the idle speed of engines that perform stratified charge combustion and uniform charge combustion such as cylinder fuel injection type engines.
In a typical engine, fuel is injected into an intake port from a fuel injection valve to charge a mixture of fuel and air to the associated combustion chamber in a uniform manner. An intake passage is opened and closed by a throttle valve, which is operated by manipulating a gas pedal. The opening of the throttle valve adjusts the intake air amount (and ultimately the amount of uniformly mixed air and fuel) that is supplied to the combustion chambers of the engine. This controls engine power.
However, when performing uniform charge combustion, a high level of negative pressure is produced by the throttling action of the throttle valve. This increases pumping loss, which is generated when the mixture is drawn into the combustion chamber from the intake port. To attempt to solve this problem, stratified charge combustion has been proposed. In stratified charge combustion, the throttle valve is opened wide, and fuel is supplied directly into each combustion chamber. This provides a mixture having a relatively low air-fuel ratio in the vicinity of the ignition plug. As a result, ignitability is enhanced.
In such an engine, each combustion chamber is provided with a uniform charge fuel injection valve and a stratified charge fuel injection valve. The uniform charge injection valve uniformly disperses fuel into the combustion chamber and the stratified charge injection valve injects fuel toward the vicinity of the ignition plug. When the engine load is small, fuel is injected from the stratified charge injection valve. Thus, the fuel is supplied in a concentrated manner about the ignition plug. The throttle valve is almost fully opened to perform stratified charge combustion. This improves fuel efficiency and decreases pumping loss.
Incidentally, several techniques for controlling idle speed of engines have been proposed. The idle speed of current engines is generally set relatively low for enhancing fuel economy. In an engine provided with an automatic transmission, shifting the selector lever from a non-drive position to a drive position increases the engine load and thus lowers the idle speed. The decrease of the idle speed is likely to stall the engine. Therefore, the idle speed controlling techniques control the engine to increase the power when the engine load is increased thereby preventing the idle speed from dropping. This stabilizes the idle speed.
Japanese Unexamined Patent Publication No. 5-39736 discloses a technique for controlling the idle speed. An idle speed controlling apparatus according to the publication inputs a signal that indicates an increase in the engine load (load signal) when a selector lever is shifted from a non-drive position to a drive position. After a predetermined period of time (delay time) has elapsed since inputting the signal, the apparatus controls the engine to increase the power. Actual increase of the engine load caused by shifting the selector lever takes place with a delay after the apparatus inputs a load signal. The apparatus of the Japanese publication is designed to deal with this delay. That is, the apparatus increases the power of the engine in accordance with the delay. Also, the apparatus changes the length of the delay time in accordance with the position of the selector lever (drive position, first gear position) when the lever is shifted from a non-drive position to a drive position.
However, employing the idle speed controlling apparatus of the above publication a stratified charge combustion engine, causes the following problems. When stratified charge combustion is performed, the throttle valve is widely open. Therefore, the power of the engine is increased by increasing the amount of fuel injection without changing the amount of intake air. Contrarily, when uniform combustion is performed, the power of the engine is increased by increasing the amount of intake air by enlarging the opening of the throttle valve (or an idle speed control valve). At this time, the amount of fuel injection is also increased for maintaining the air-fuel ratio.
As described above, the engine power is increased differently when stratified charge combustion is performed from when uniform charge combustion is performed. Accordingly, when the engine is commanded to increase the power, the time lag between inputting the command signal and the actual power increase is different when stratified charge combustion is performed from when uniform charge combustion is performed. Specifically, when stratified charge combustion is performed, the power increase of the engine is completed in a relatively short period of time after the command signal is inputted to the engine because only the injection fuel amount into the combustion chambers is increased. On the other hand, when uniform charge combustion is performed, the opening of the throttle valve (or the opening of the idle speed control valve) is increased and then the increased amount of air, which corresponds to the increase of the valve's opening, must flow into the combustion chamber from the throttle valve. Therefore, the time period between inputting a command for increasing power and the resulting power increase is longer in uniform charge combustion than in stratified charge combustion.
If the time period between the input of a command for increasing engine power and the resulting increase of the engine power is short, the engine power may be increased before engine the load is increased. In this case, the idle speed of the engine is increased, and the subsequent increase of the engine load results in a shock to the engine. This causes the idle speed to drop abruptly. If the time period is excessively long, on the other hand, the engine load may be increased before the power of the engine is increased. The increased load causes the idle speed to drop before the engine power is increased. This may stall the engine.